Many modern cellular communication systems such as High Speed Packet Access and Long Term Evolution (HSPA and LTE, respectively, in the following), as specified by the 3rd Generation Partnership Project (3GPP in the following) use automatic link adaptation to achieve efficient communication under varying transmission conditions. The effective bit rate is varied quickly (along with related transmission parameters such as code rate and modulation scheme) depending on the predicted radio conditions. When the radio conditions get worse, the bit rate is decreased to reduce the probability of decoding error; when the conditions improves, the bit rate is increased to increase the transmission efficiency without causing a too high error probability. The radio conditions are often predicted based on past measurements of the radio channel.
An often used protocol in these systems is Hybrid Automatic Repeat Request, in the following referred to as HARQ. With HARQ, failure to decode a received transport block results in a retransmission, possibly with a different redundancy version.
When a receiver fails in its attempt to decode a transport block, it typically stores the received signal or a processed version of the signal, and combines it with a later received signal for a retransmission of that block. This is known as soft combining, variants of which include Chase combining and incremental redundancy. Soft combining greatly increases the probability of a correct decoding.
In many HARQ protocols, the receiver sends a HARQ feedback after each decoding attempt, in the form of a positive or negative acknowledgement (in the following referred to as ACK and NACK, respectively) to indicate if the particular transport block was correctly decoded or not. In case a NACK is sent, the transmitter typically retransmits the transport block. In the case of ACK, the transmitter may instead use its resources to transmit new data, to the same or a different receiving entity.
Another way of using the HARQ protocol is to let the receiving entity control the transmissions, as is done on the LTE uplink where the receiving entity is a NodeB unit and the transmitting entity is a user equipment (UE in the following). The receiving entity sends a grant for each requested transmission, indicating among other things the transport format (i.e. modulation and code rate) and whether a retransmission or an original transmission is requested. A grant for a retransmission may in some cases consist of a single bit, similar to a HARQ ACK/NACK, but a retransmission may in other cases be a complete grant of the same size as a grant for an original transmission. With this view, transmission grants may be seen as a kind of HARQ feedback.
In both HARQ variants, the receiving entity must perform a complete decoding attempt before it can decide upon its next action. However, modern error-correcting codes, such as Turbo codes, are very complex to decode, resulting in long delays from transmission until a message can be sent back to the transmitting entity. This results in long round-trip delays of the HARQ retransmissions. For example, in LTE the minimum round-trip delay is 8 ms.
HARQ with soft combining may be viewed as a kind of (implicit) link adaptation mechanism. This is the case if the bit rate is chosen so high that one or more retransmissions are often needed. The effective bit rate of an entire transmission of a transport block then depends on the total number of transmissions, including original transmissions and retransmissions, as well as transport format parameters such as modulation and code rate. Contrary to link adaptation based on past measurements of radio conditions, the effective bit rate of such a HARQ transmission is determined by the radio conditions during the actual transmission of the transport block.
Because of the nature of radio channels, and the behavior of interference from other transmitters, it is difficult to make an accurate prediction of the radio conditions for a particular transmission. This has made it necessary, in the prior art, to apply a significant margin against sudden variations, in order to keep the probability of decoding error acceptably low. Such a margin reduces the average throughput.
A higher average throughput may theoretically be achieved by using a higher original transmission bit rate and accepting a higher error probability. However, an acceptance of a higher error probability also implies an acceptance of a larger number of retransmissions in the HARQ procedure. A drawback with such an approach is that it results in much longer packet delays, since each retransmission adds at least 8 ms to the total transmission time of the packet, in case of an LTE system.